User equipment (UE), also known as mobile stations, wireless terminals and/or mobile terminals are enabled to communicate wirelessly in a wireless communication system, sometimes also referred to as a cellular radio system. The communication may be made e.g. between two user equipment units, between a user equipment and a regular telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more core networks. The user equipment units may further be referred to as mobile telephones, cellular telephones, laptops with wireless capability. The user equipment units in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another user equipment or a server.
The wireless communication system covers a geographical area which is divided into cell areas, with each cell area being served by a network node, or base station e.g. a Radio Base Station (RBS), which in some networks may be referred to as “eNB”, “eNodeB”, “NodeB” or “B node”, depending on the technology and terminology used. The network nodes may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the network node/base station at a base station site. One base station, situated on the base station site, may serve one or several cells. The network nodes communicate over the air interface operating on radio frequencies with the user equipment units within range of the respective network node, i.e. within the cell served by the network node. It is to be noted that the expression (serving) cell sometimes is utilised when referring to the carrier serving the user equipment, which may be the case also in this disclosure. Also the term component carrier is used interchangeable for cell in this context. Among the serving cells of a terminal operating with carrier aggregation there is always a primary cell or component carrier—PCell—and one or multiple secondary cells or component carriers—SCell.
In some radio access networks, several network nodes may be connected, e.g. by landlines or microwave, to a Radio Network Controller (RNC) e.g. in Universal Mobile Telecommunications System (UMTS). The RNC, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural network nodes connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), network nodes, or base stations, may be referred to as eNodeBs or even eNBs. They may in turn be connected to one or more core networks, possibly via a gateway e.g. a radio access gateway. The 3GPP is responsible for the standardization of LTE, and is continuously developing and updating new releases of LTE, such as e.g. LTE Rel. 8 and LTE Rel. 10.
LTE is a technology for realizing high-speed packet-based communication that may reach high data rates both in the downlink and in the uplink, and is thought of as a next generation mobile communication system relative to UMTS.
In the present context, the expressions downlink, downstream link or forward link may be used for the transmission path from the network node to the user equipment. The expression uplink, upstream link or reverse link may be used for the transmission path in the opposite direction i.e. from the user equipment to the network node.
LTE uses Orthogonal Frequency-Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT)-spread OFDM in the uplink. The basic LTE downlink physical resource may thus be seen as a time-frequency grid as illustrated in FIG. 1A, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.
In the time domain, LTE downlink transmissions are organised into radio frames of 10 ms, each radio frame comprising ten equally-sized subframes of 1 ms, which is illustrated in FIG. 1B.
Furthermore, the resource allocation in LTE is typically described in terms of Resource Blocks (RB), where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
The LTE Rel-10 specifications have recently been standardized, supporting Component Carrier (CC) bandwidths up to 20 MHz (which is the maximal LTE Rel-8 carrier bandwidth). Hence, an LTE Rel-10 operation wider than 20 MHz is possible and appear as a number of LTE carriers to an LTE Rel-10 terminal.
In particular for early LTE Rel-10 deployments it may be expected that there will be a smaller number of LTE Rel-10-capable terminals compared to many LTE legacy terminals. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e. that it is possible to implement carriers where legacy terminals may be scheduled in all parts of the wideband LTE Rel-10 carrier. The straightforward way to obtain this would be by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 terminal may receive multiple CC, where the CC have, or at least the possibility to have, the same structure as a Rel-8 carrier. CA is illustrated in FIG. 1C.
During initial access an LTE Rel-10 terminal behaves similar to an LTE Rel-8 terminal. Upon successful connection to the network a terminal may—depending on its own capabilities and the network—be configured with additional CCs in the UL and DL. Configuration is based on Radio Resource Control (RRC). Due to the heavy signalling and rather slow speed of RRC signalling it is envisioned that a terminal may be configured with multiple CCs even though not all of them are currently used. If a terminal is activated on multiple CCs this would imply it has to monitor all DL CCs for PDCCH and PDSCH. This implies a wider receiver bandwidth, higher sampling rates, etc. resulting in high power consumption.
In order to preserve the orthogonality in UL the UL transmissions from multiple UEs need to be time aligned at the eNodeB. Since UEs may be located at different distances from the eNodeB, see FIG. 2, the UEs would in such case need to initiate their UL transmissions at different points in time. A UE far from the eNodeB needs to start transmission earlier than a UE close to the eNodeB. This may for example be handled by Timing Advance (TA) of the UL transmissions, i.e. that a UE starts its UL transmission before a nominal time given by the timing of the DL signal received by the UE. This concept is illustrated in FIG. 3, in which timing advance of UL transmissions, depending on distance to eNodeB is illustrated.
The UL timing advance is maintained by the eNodeB through timing alignment commands to the UE based on measurements on UL transmissions from that UE.
Through timing alignment commands, the UE is ordered to start its UL transmissions earlier or later. This applies to all UL transmissions except for random access preamble transmissions on Physical Random Access Channel (PRACH), i.e. comprising transmissions on Physical Uplink Shared Channel (PUSCH), Physical Uplink Control Channel (PUCCH), and Sounding Reference Signal (SRS).
There is a strict relation between DL transmissions and the corresponding UL transmission. Examples of this are the timing between a DL-SCH transmission on Physical Downlink Shared Channel (PDSCH) to the HARQ ACK/NACK feedback transmitted in UL (either on PUCCH or PUSCH). Yet such an example is the timing between an UL grant transmission on Physical Downlink Control Channel (PDCCH) to the UL-SCH transmission on PUSCH.
By increasing the timing advance value for a UE, the UE processing time between the reception of the UL grant in DL and the corresponding UL transmission decreases. For this reason, an upper limit on the maximum timing advance has been defined by 3GPP in order to set a lower limit on the processing time available for a UE. For LTE, this value has been set to roughly 667 μs which corresponds to a cell range of 100 km. Note that the TA value compensates for the round trip delay.
In LTE Rel-10 there is only a single timing advance value per UE and all UL cells are assumed to have the same transmission timing. The reference point for the timing advance is the receive timing of the primary DL cell.
In LTE Rel-11 different serving cells used by the same UE may have different timing advance. Most likely the serving cells sharing the same TA value (for example depending on the deployment) will be configured by the network to belong to a so called TA group. If at least one serving cell of the TA group is time aligned, all serving cells belonging to the same group may use this TA value. To obtain time alignment for an SCell belonging to a different TA group than the PCell, the current 3GPP assumption is that network initiated random access may be used to obtain initial TA for this SCell (and for the TA group the SCell belongs to).
In LTE, as in any communication system, a mobile node may need to contact the network (via the eNodeB) without having a dedicated resource in the Uplink (from UE to base station). To handle this, a random access procedure is available where a UE that does not have a dedicated UL resource may transmit a signal to the base station. The first message of this procedure is typically transmitted on a special resource reserved for random access, a physical random access channel (PRACH). This channel can for instance be limited in time and/or frequency (as in LTE). FIG. 4 is a principal illustration of random-access-preamble transmission. The resources available for PRACH transmission is provided to the terminals as part of the broadcasted system information (or as part of dedicated RRC signalling in case of e.g. handover).
Signalling over the air interface for the contention-based random access procedure used in LTE is illustrated in FIG. 5. The UE starts the random access procedure by randomly selecting one of the preambles available for contention-based random access. The UE then transmits the selected random access preamble on the physical random access channel (PRACH) to eNodeB in the Radio Access Network (RAN). The RAN acknowledges any preamble it detects by transmitting a random access response (MSG2) The MSG2 is transmitted in the DL to the UE.
When receiving the response the UE uses the grant comprised in MSG2 to transmit a message (MSG3) that in part is used to trigger the establishment of radio resource control and in part to uniquely identify the UE on the common channels of the cell. MSG4 which is the contention resolution message is transmitted from the network to the UE.
The procedure ends with RAN solving any preamble contention that may have occurred for the case that multiple UE transmitted the same preamble at the same time.
The UE may also perform non-contention based random access. A non-contention based random access or contention free random access can e.g. be initiated by the eNB to get the UE to achieve synchronisation in UL. The eNB initiates a non-contention based random access either by sending a PDCCH order or indicating it in an RRC message. The later of the two is used in case of HandOver (HO).
Similar to the contention based random access the MSG2 is transmitted in the DL to the UE. The UE considers the contention resolution successfully completed after it has received MSG2 successfully.
For the contention free random access as well as for the contention based random access, the MSG2 contains a timing alignment value. This enables the eNB to set the initial/updated timing of subsequent UL transmissions.
In LTE the Rel-10 random access procedure is limited to the primary cell only. This implies that the UE may only send a preamble on the primary cell. Further MSG2 and MSG3 are only received and transmitted on the primary cell. MSG4 may however in Rel-10 be received on any DL cell.
In LTE Rel-11, the current assumption (RAN2#75, August 2011) is that the random access procedure will be supported also on secondary cells, at least for the UEs supporting Rel-11 carrier aggregation. So far only network initiated random access on SCells is assumed.
To enable frequency-domain processing of RA reception LTE uses RA preambles with a cyclic prefix. The cyclic prefix should cover maximum Round Trip Time, RTT, in the cell plus maximum expected delay spread. To be able to cope with a wide variety of deployment scenarios LTE defines 5 different preamble formats, which are illustrated in FIG. 6.
RA preamble format 4 only has a cyclic prefix length of approximately 15 μs. Assuming a maximum delay spread of 5 μs the maximum supported RTT becomes approximately 10 μs.
In LTE Rel-11 it has been agreed that UEs should support to simultaneously aggregate cells from network nodes at different physical locations, even for UL. This requires the UEs to have multiple TA values due to different propagation delays in the UL. A TA value needs a timing reference. A UE starts UL subframe transmission to cell i at TAi seconds earlier than the timing reference for cell i.
Which timing reference should be used for SCells in Rel-11 is currently discussed in 3GPP RAN2 and alternatives brought up are the PCell or an SCell. In Rel-10 the PCell downlink is used as timing reference for all cells.
Technical advantages for using the PCell downlink as timing reference are, for example, that Radio Link Monitoring (RLM) is only done for the PCell which makes the timing reference reliable. Also, if an SCell is used as timing reference, complexity is needed to handle what should happen when this SCell is deactivated; this would be avoided if the PCell is used as timing reference since the PCell cannot be deactivated. Aside from these technical aspects it is strived for to align behaviour between LTE releases.
However, it has been identified that if the PCell downlink is used as timing reference for an SCell configured with preamble format 4 the RA preamble could reach the SCell receiver outside of the preamble reception window and the RA procedure then fails as the SCell receiver does not detect/receive the preamble signalling from the UE.